Changes impact theme-twisty icons in the TreeView component:
- Make sure theme-twisty uses the selection color (white) in selected rows;
but override that style for Network details accordion headers.
- Improve the vertical alignment and pixel-perfect icon rendering by
making sure the icon element takes the full available height (16px)
and avoiding assymetrical padding (affects the rendering of centered
background-image, making them blurry on 1x displays).
- Tweak the horizontal spacing of theme-twisty icons so that they take
up a total of 20px width (similar to what is used in Debugger and
Inspector), versus 17px previously.
Differential Revision: https://phabricator.services.mozilla.com/D29550
